A variety of printing systems can apply or print images (e.g., graphics, text, or the like) on exterior surfaces of objects. Many of these systems directly engage or contact the exterior surfaces on which the images are printed. For example, a thermal transfer printing process can involve a print ribbon having ink that is transferred onto a target object by applying heat and pressure to the print ribbon as the print ribbon engages the target object. The print ribbon may be provided from a first spindle, where the print ribbon extends through one or more rollers to cause the ribbon to be disposed between a thermal print head and the target object. The first spindle now uses a friction brake in the form of a cord. It could also be a motor driven at low power, or one that is connected to a regenerative circuit. From this location, the print ribbon may extend through one or more other rollers to a second spindle. The second spindle may apply a torque to cause the print ribbon to unwind from the first spindle and move between the thermal print head and the target object, as well as move relative to the thermal print head and the target object, as the thermal print head applies heat and pressure to transfer ink from the print ribbon onto the target object.
The print ribbon is initially wound up on the first spindle such that the wound up print ribbon has an outer diameter. As the print ribbon is unwound from the first spindle, this outer diameter decreases. Changes in this outer diameter can result in changes in the tension in the print ribbon as the print ribbon moves between the thermal print head and the target object. Additionally, in some instances, the second spindle may apply a torque that is intermittent rather than continuous. For example, the torque applied by the second spindle to pull the print ribbon through the printing system may not be constant with respect to time. As a result, the tension in the print ribbon can change. If the tension changes too rapidly, the first spindle from which the print ribbon is taken may continue to spin somewhat uncontrollably such that the print ribbon continues to unwind from the first spindle even through the second spindle is no longer applying torque to the print ribbon.
These changes in the tension in the print ribbon can cause problems with transferring the ink from the print ribbon to the target object. For example, the images printed from the print ribbon can become blurry or incomplete.